The dsDNA bacteriophages provide an unparalleled resource for investigating the principles of biological assembly at the molecular level. This is a proposal to continue the study of these problems using the E. coli bacteriophages lambda and HK97. Specific interests include assembly of the heads of HK97, with an emphasis on the extensive conformational rearrangement in the capsid protein that happens during DNA packaging, assembly of the phage lambda tail, with an emphasis on the newly discovered chaperone-mediated step in the assembly of the tail length template, and assembly and structure of the lambda long tail fibers. Information gained in this study will be generalizable beyond bacteriophages and will therefore affect our understanding of, among others, assembly processes in animal and plant viruses, which are possible points for anti-viral intervention.